The US has the highest level of wealth inequality in the world, so assuming that the ultra-rich donate more, their impact should be higher in the US than anywhere else.
I think I want income inequality. Wealth inequality might be totally meaningless as wealth is guesstimate not exact value usually, e.g. look here https://en.wikipedia.org/wiki/List_of_countries_by_distribut..., is hard to say anything at all from Gini Wealth. Zimbabwe and Denmark can be compared by Gini index but that's completely nonsensical result: one corrupt and one happiest country in the world.
I have mentioned law of large numbers because of population size, however rich ultra-rich might be that can't skew results in USA.
Are you saying this doesn't also occur in other nations, like Germany?